The present invention relates to a method for obtaining sunscreen compositions with improved photostability in both the UVA and UVB regions, said method improves photostability by the encapsulation of sunscreen active ingredients (which are photodegraded when present together in the same sunscreen composition) in separate microcapsules and by incorporating said microcapsules into any acceptable vehicle.
With the growing demand for higher SPF values and for broad-spectrum protection, manufacturers are forced to combine several active ingredients hence the problem of photo-induced cross reactivity between sunscreen active ingredients is becoming more severe.
For example, butyl methoxydibenzoylmethane (other known names: 4-tert-butyl-4xe2x80x2-methoxydibenzoylmethane, Avobenzone, BMDBM) was recently listed as category 1 in FDA""s Final Monograph for OTC Sunscreen Products. However, this excellent UVA absorber presents a number of formulation problems. (W. Johncock, Sun Protection The Influencing Factors in Creating Effective Products, March 1999 London, March 1999 and references therein). A major problem of this sunscreen compound is the photoinduced interaction with the most widely used UVB absorber octylmethoxycinnamate (R. M. Sayre et. al, 26th Annual Meeting if the American Society for Photobiology, Abstr. No. SPM-A7). This cross reactivity contributes significantly to the photochemical instability of both the UVB and the UVA active ingredients. The photostability issue when combining UVA with UVB filters seems to be more general than the case of BMDBM and OMC. In the same paper by Johncock, he reveals that new UVA chromophors examined as candidates for new products have been discarded as they were found to interact with OMC in an unacceptable way.
Photoinduced interactions of BMDBM restrict also the incorporation of this absorber with physical sunscreen agent such as titanium dioxide. In the US, formulation of BMDBM together with TiO2 or ZnO is not allowed.
Several patents, for example WO94/04131 and EP780119, have been published regarding the improved photostability of BMDBM in the presence of other sunscreen ingredients such as Octocrylene, p-methyl-benzylidene camphor and others, at certain molar ratios. These patents, however, do not mention cinammic acid derivatives and in particularly do not mention OMC, which is currently the most widely used UVB absorber. In practice, many popular products of broadband light screening compositions contain OMC and BMDBM as the main UV absorbers. Unfortunately, the photostability of many such products is poor, and stabilization by Octocrylene or p-methyl-benzylidene camphor is insufficient.
Patents EP 0 920 858 A2 and U.S. Pat. No. 5,985,251 teach that improved photostability of compositions based on benzylidene derivatives and cinnamic acid derivatives may be obtained by using a water soluble cinnamic acid derivative along with a fat soluble benzylidene derivative. Assuming that photo-instability results from a diffusion controlled reaction between exited species of the sunscreen agents, it may be rationalized that the separation of the two incompatible actives into different phases enhances stability by creating a diffusional barrier.
The method disclosed here teaches that improved photostability is achieved by creating an efficient diffusional barrier through effective encapsulation of at least one of two incompatible sunscreen actives, as in the case of cinnamic derivatives and dibenzoylmethane derivatives. In a preferred embodiment of this invention, both incompatible sunscreen actives are encapsulated separately.
The method presented in this invention is not limited to any specific combination of UVB and UVA absorbing ingredients, and affords further improvement in the photostability over prior stabilization methods.
According to the method of the present invention there is no formulation restriction concerning the ratio between UVA and UVB absorbers, thus any desired UVA/UVB protection ratio can be achieved.
The present invention provides a method for obtaining improved photostability of a sunscreen composition that contains at least two sunscreen active ingredients, which are photo-unstable when formulated together, comprising
(a) microencapsulating at least one of said active ingredients in an encapsulating material suitable for holding the encapsulated active ingredient material, thus reducing or preventing its leaching out of the capsules; and
(b) adding other acceptable components and additives needed for the preparation of said composition.
According to the invention the sunscreen active ingredients are selected from the group consisting of UVA absorbers and UVB absorbers, preferably encapsulated in sol-gel microcapsules.
In one embodiment the improved photostability of a sunscreen composition can be obtained by encapsulating each of the active ingredients in separate sol-gel microcapsules.
In another embodiment at least one of the sunscreen active ingredient is not encapsulated.
According to the present invention one of the sunscreen active ingredients can be a dibenzoylmethane derivative, preferably selected from a group consisting of 4-tert-butyl-4xe2x80x2-methoxydibenzoylmethane, 2-methyldibenzoylmethane, 4-methyl-dibenzoyl-ethane, 4-isopropyldibenzoyl-methane, 4-tert-butyldibenzoylmethane, 2,4-dimethyldibenzoylmethane, 2,5-dimethyldibenzoylmethane, 4,4xe2x80x2-diisopropyl-dibenzoylmethane, 2-methyl-5-isopropyl-4xe2x80x2-methoxydibenzoylmethane, 2-methyl-5-tert-butyl-4xe2x80x2-methoxy-dibenzoylmethane, 2,4-dimethyl-4xe2x80x2-methoxydibenzoylmethane and 2,6-dimethyl-4-tert-butyl-4xe2x80x2-methoxy-dibenzoylmethane.
According to one preferred embodiment the sunscreen active ingredients are 4-tert-butyl-4xe2x80x2-methoxydibenzoylmethane and 2-ethylhexyl 4-methoxycinnamate.
According to another preferred embodiment the sunscreen active ingredients are 2-ethylhexyl p-methoxycinnamate and cinoxate.
The 4-tert-butyl-4xe2x80x2-methoxydibenzoylmethane can be co-encapsulated with homosalate, with 2-cyano-3,3-diphenylacrylic acid 2-ethylhexyl ester, or with a suitable cosmetic oil such as dicaprylyl maleate, Capric triglyceride, caprylic triglyceride, octyl palmitate, C12-C15 alkyl benzoate, dioctyl maleate, dioctyl malate, propylene glycol dicaprylate, propylene glycol dicaprate, diisopropyl adipate, hexyl laurate, and mixtures thereof.
The composition of the present invention can further include a physical sunblock active ingredient selected from the group consisting of titanium dioxide, zinc oxide, iron oxide, and mixtures thereof, wherein the physical sunblock ingredient can be of any commercially available grade, including surface treated particles such as titanium dioxide particles which have been surface treated with silica, alumina, stearic acid or by any other surface treatment.
The sunscreen composition of the present invention can be incorporated into a cosmetically acceptable formulation for use for the protection of human epidermis or hair against UV radiation.
The sunscreen compositions can be in the form of a lotion, a cream, a milk, a gel, an oil, an aerosol, a spray, a foam, a solid stick, a powder, a shampoo, a hair conditioner, a lacquer or a make up.